(1) Field of the Invention
This invention relates to power-assist vehicles, and more particularly to power-assist wheelchairs.
(2) Description of the Related Art
A wide variety of wheelchair configurations exist. Wheelchairs generally allow a seated occupant who has little or no use of his legs to navigate from one place to another. Wheelchairs are commonly configured to be powered in one or more ways. Many wheelchairs have handles at their back to allow an attendant to push the wheelchair. Many wheelchairs may be occupant-powered, typically having a large diameter drive wheel at each side, each drive wheel having a concentric handrim which may be gripped by the occupant to rotate the drive wheel to drive the wheelchair. Motor-driven wheelchairs may be used where the occupant is unable to power the wheelchair himself. In some motor-driven wheelchairs, two electric motors are respectively coupled to the left and right drive wheels. The motors may be controlled by a joystick which can drive the wheels at different speeds to provide steering. One example of a motor-driven wheelchair with a suspension mechanism is shown in U.S. Pat. No. 4,339,013 of Gerald I. Weigt.
The ""124 application discloses a power-assist wheelchair. Another power-assist wheelchair is disclosed in U.S. Pat. No. 5,818,189 (the ""189 patent), the disclosure of which is incorporated by reference in its entirety herein. In a power-assist wheelchair, motor power supplements power provided by the occupant. The ""124 application and ""189 patent teach a motor associated with each drive wheel applying power to the drive wheel responsive to the torque applied by the occupant to the associated handrim. Electricity to power the motors may be provided one or more rechargeable batteries.
In addition to the drive wheels, wheelchairs commonly include at least one additional wheel. In a common wheelchair configuration such as illustrated in the ""124 patent, there are two relatively small caster-like front wheels which freely pivot about generally vertical caster axes to permit the wheelchair to turn. Some racing wheelchairs, however, feature a single central front wheel while other wheelchairs locate the additional wheel(s) behind the drive wheels.
In one aspect the invention is directed to a power assist vehicle. The vehicle has a drive wheel and input means for receiving a driving torque from a user. A transmission assembly couples a motor to the drive wheel to permit the motor to drive the drive wheel and couples the input means to the drive wheel so as to permit the input means to simultaneously drive the drive wheel. A control system is programmed with at least one control map defining a desired dynamic of the vehicle and programmed with a desired mass-indicative parameter and a desired drag parameter.
The control system advantageously utilizes the control map and measured values of driving torque from the user and an actual velocity to determine a desired velocity and employs a single-axis velocity servo loop to control the motor to reduce an error component between the actual velocity and the desired velocity. The desired velocity may be produced in response to an iterative integration of: measured user-applied torque multiplied by a first constant; and the negative of the sign of the desired velocity multiplied by a second constant. The desired velocity may be produced in response to an iterative integration of: measured user-applied torque multiplied by a first constant; and a damping effect provided by a function of the desired velocity.
Aspects of the invention may be implemented in a wheelchair having a pair of drive wheels and handrims.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.